Little research has been conducted on the incidence of abnormal movements characterized as dyskinesia in the institutionalized mentally retarded. An instrument has been developed to examine dyskinesia in this population - the Dyskinesia Identification System - Coldwater (DIS-CO). The instrument will be evaluated for interrater agreement, test-retest reliability or stability, and agreement between facilities. With trained raters using DIS-CO, all residents of a retardation facility will be surveyed for dyskinesia, regardless of drug status. The incidence of abnormal movements will then be compared in residents who have never received psychotropic medication, residents who have received such drugs but are not currently receiving them, and residents who are receiving psychotropic drugs. Using DIS-CO we will follow the appearance and course of abnormal movements in residents whose medication is being decreased or withdrawn; we will also examine changes in ward and school behavior as drug doses decrease, using the recently developed Resident Behavior Rating Scale (RBRS).